We are studying the hydrolysis of ATP catalyzed by ArsA ATPase to see if it catalyses an phosphate-water oxygen exchange analogous to that of mitochonrial F1 ATPase. Studies on the latter have provided important information on the relation between ATP hydrolysis and energy coupling and on site-site interactions (P. Boyer's binding change mechanism). No exchange was detected at high concentrations of ATP and we are now using low levels of ATP to try to examine ATP produced by single site turnover. This is now possible because of the availability of larger amounts of purified ArsA protein.